1. Field of the Invention
The present invention relates to a developing apparatus to be used in an image forming apparatus such as a copying machine and a laser beam printer using an electrostatic recording process or an electrophotographic process, and configured to develop an electrostatic image formed on an image bearing member with developer including toner and carrier.
2. Description of the Related Art
Hitherto, in image forming apparatus such as a copying machine, an electrostatic latent image formed on a photosensitive drum serving as an image bearing member is developed into a visible image by a developing apparatus.
The developing apparatus is configured to supply toner to the photosensitive drum so as to visualize the latent image as a toner image. A developing apparatus using single-component developer including magnetic toner as developer has been employed, and a developing apparatus using two-component developer obtained by mixing non-magnetic toner and magnetic carrier together has also been employed in many cases. Development of the latent image with the two-component developer is excellent in stability of a toner charge amount, and hence color images can be formed with excellent color tone. For this reason, the two-component developer is suited to use in a color-image forming apparatus.
The developing apparatus includes a regulating blade configured to regulate an amount of developer to be coated onto a developing sleeve serving as a developer carrying member of the developing apparatus. Examples of such a developing apparatus include a developing apparatus of a so-called upper regulation type in which the regulating blade is positioned above the developing sleeve. In this case, there is often employed such a configuration that the developing sleeve is rotated from top to bottom at a facing portion between the developing sleeve and the photosensitive drum.
In addition, there has been proposed a developing apparatus of a so-called lower regulation type in which the regulating blade is positioned below the developing sleeve. In this case, there is often employed such a configuration that the developing sleeve is rotated from bottom to top at the facing portion between the developing sleeve and the photosensitive drum.
In a case where the developing apparatus of the above-mentioned upper regulation type is employed for the photosensitive drum to be rotated from top to bottom at the facing portion between the photosensitive drum and the developing sleeve, the developing sleeve and the photosensitive drum are rotated in the same direction at the facing portion. Thus, such a blank area phenomenon may occur that toner does not adhere to a boundary part between a halftone image portion and a solid image portion. On the other hand, in a case where the developing apparatus of the lower regulation type is employed, the developing sleeve and the photosensitive drum are rotated in directions reverse to each other at the facing portion. Thus, there is an advantage in that the blank area phenomenon does not occur. For this reason, in a case where the blank area phenomenon is conspicuous, such as a case where carrier having relatively high resistance is used, the developing apparatus of the lower regulation type is frequently employed.
Further, the developing apparatus of the above-mentioned lower regulation type is frequently employed, for example, in a case where an intermediate transfer belt is arranged above the photosensitive drum. Specifically, the developing apparatus of the above-mentioned lower regulation type is employed so that the developing sleeve and the photosensitive drum are rotated in the same direction at the facing portion in a case where the photosensitive drum is rotated from bottom to top at the facing portion between the photosensitive drum and the developing sleeve.
The developing apparatus of the above-mentioned lower regulation type is disclosed, for example, in Japanese Patent Application Laid-Open No. H11-143231. FIG. 5 illustrates this developing apparatus.
In a developing apparatus 102 of the above-mentioned lower regulation type, a developing sleeve 120 is provided in a rotatable manner at an opening portion of a developing container 110 containing two-component developer. The developing sleeve 120 is arranged so as to face a photosensitive drum 28, and a cylindrical magnet 121 is arranged in an unrotatable manner at the inside of the developing sleeve 120. At a facing portion between the developing sleeve 120 and the photosensitive drum 28, the developing sleeve 120 is configured to be rotated in a direction reverse to that of the photosensitive drum 28 to be rotated in the direction indicated by the arrow so that the developer attracted and held on a surface of the developing sleeve 120 is conveyed to a developing portion facing the photosensitive drum 28. The magnet 121 has a developing magnetic pole N1 on the side of the photosensitive drum 28. The magnet 121 also has a first conveying magnetic pole S3, a second conveying magnetic pole N2, a stripping magnetic pole S2, and a scooping magnetic pole S1, which serves as a repulsive magnetic pole cooperatively with the stripping magnetic pole S2. Those magnetic poles are formed along a rotation direction of the developing sleeve 120 in an order from the developing magnetic pole N1.
The developer contained in the developing container 110 is carried onto the developing sleeve 120 by an action of the scooping magnetic pole S1 at a position (scooping position) Q on the surface of the developing sleeve 120, which corresponds to the scooping magnetic pole S1 of the magnet 121. The developer carried on the developing sleeve 120 is regulated in layer thickness by a developing blade 122, and then reaches the developing portion. At the developing portion, a magnetic brush is formed by an action of the developing magnetic pole N1 so that the latent image on the photosensitive drum 28 is developed.
The developer on the developing sleeve 120, which is reduced in toner density as a result of the development, is returned into the developing container 110 by being held on the developing sleeve 120 by actions of the first conveying magnetic pole S3 and the second conveying magnetic pole N2. Then, at another position (developer falling position) D on the surface of the developing sleeve 120, at which a magnetic flux density between the stripping magnetic pole S2 and the scooping magnetic pole S1 is low, the developer on the developing sleeve 120 is released to fall. At the scooping position Q, the developing sleeve 120, from which the developer is released, is re-supplied with the developer as described above.
At a position below the developing sleeve 120 inside the developing container 110, a developing chamber 113 including a first agitating-and-conveying member 123 is arranged, and an agitating chamber 114 including a second agitating-and-conveying member 124 is also arranged through intermediation of a partition wall 140. The first agitating-and-conveying member 123 and the second agitating-and-conveying member 124 are each a screw type member, and generally include a helical screw blade 128.
The developer on the developing sleeve 120, which is reduced in toner density, is released to fall onto the first agitating-and-conveying member 123 inside the developing chamber 113. This developer is agitated by the first agitating-and-conveying member 123, and then re-supplied onto the developing sleeve 120 by an action of the scooping magnetic pole S1 of the magnet 121.
At this time, after the developer on the developing sleeve 120, which is reduced in toner density as a result of the development, is released from the developing sleeve 120, the developer may be re-supplied onto the developing sleeve 120 without being sufficiently agitated by the first agitating-and-conveying member 123. In this case, the latent image on the photosensitive drum 28 is developed with the toner which is reduced in toner density. For this reason, image density becomes uneven, resulting in adverse effects such as uneven shading.
As a countermeasure, normally, as illustrated in FIG. 5, in order that the developer released from the developing sleeve 120, which is reduced in toner density, be reliably subjected to an agitating action by the first agitating-and-conveying member 123, the first agitating-and-conveying member 123 and the developing sleeve 120 are rotated in directions reverse to each other at a facing portion therebetween. Further, the developing sleeve 120 is arranged in a manner that a position vertically below the position (scooping position) Q on the surface of the developing sleeve 120, which corresponds to the scooping magnetic pole S1 of the magnet 121, is closer to the developing blade 122 with respect to an axis of the first agitating-and-conveying member 123.
As a configuration for preventing the uneven shading which may be caused by the developer released from the developing sleeve and re-supplied to the developing sleeve without being sufficiently agitated, there has been proposed a developing apparatus of a separate function type as disclosed in Japanese Patent Application Laid-Open No. 2012-42737. The developing apparatus according to Japanese Patent Application Laid-open No. 2012-42737 is an example of a combination of the developing apparatus of the above-mentioned lower regulation type and the separate function type. FIG. 6 illustrates this configuration.
A developing apparatus 31 of the separate function type refers to a developing apparatus of a developer circulation type, in which developer is supplied onto a developing sleeve 3 from a developing chamber 11 including a first agitating-and-conveying member 13, and after the development, the developer is collected into an agitating chamber 12 including a second agitating-and-conveying member 14 without being returned into the developing chamber 11. By employing the separate function type, the developer that is reduced in toner density as a result of the development is prevented from being re-supplied to the developing sleeve 3 without being sufficiently agitated. As a result, the problems such as the above-mentioned uneven shading can be prevented.
However, the developing apparatus 31 of the separate function type as disclosed in Japanese Patent Application Laid-Open No. 2012-42737 has a problem in that a surface of the developer cannot be evenly spread. The problem is caused due to the collection of the developer from the developing sleeve 3 into the agitating chamber 12. Normally, in a configuration for circulating developer in a looped manner between the developing chamber and the agitating chamber, the developer flows only in one direction. Thus, an even amount of the developer can be relatively easily distributed by appropriately designing developing screws. However, in the configuration of the developing apparatus 31 of the separate function type, the developer on the developing sleeve 3 is transported along a longitudinal direction of the agitating chamber 12. Thus, in the developing chamber 11, the developer decreases in amount toward a downstream side in a conveying direction along with supply to the developing sleeve 3. On the other hand, in the agitating chamber 12, the developer increases in amount toward the downstream side in the conveying direction. Thus, the even amount of the developer is difficult to distribute in a longitudinal direction of a developing container 2.
In this way, in the developing apparatus of the separate function type, the developer is liable to stagnate on the downstream side in the agitating chamber and the upstream side in the developing chamber. Thus, the surface of the developer contained in the developing chamber becomes lower from the upstream side toward the downstream side. As a result, an amount of the developer to be supplied to the developing sleeve on the downstream side in the developing chamber decreases. In particular, in the developing apparatus of the lower regulation type, in which the blade configured to regulate a coating amount of the developing sleeve is positioned below the developing sleeve, depending on a position of the scooping magnetic pole S1 of the magnet arranged at the inside of the developing sleeve, coating failure may be caused by influence of the lowered surface of the developer.